This invention relates to an optical disc recording device of a mark length recording type which records data by forming pits on a disc with laser beam. The optical disc recording device is capable of reducing jitter occurring in a reproduced signal by reducing error in pit length (length of a pit in the advancing direction of a beam spot) or blank length (length of a blank portion of the disc between adjacent pits in the advancing direction of the beam spot) thereby improving the quality of the reproduced signal owing to improvement of signal-to-noise ratio.
For recording data on a master disc of a Compact Disc (CD) or a video disc or a DRAW disc which can be used as a document filing disc memory by writing data thereon, a disc on which recording film made of tellurium or bismuth is uniformly coated is rotated at a constant revolution velocity or constant linear velocity and pits are formed by melting the recording film with laser beam. This recording has generally been performed by irradiating laser beam for a period of time corresponding to length of a pit to be formed (e.g., 231 nsec for 1T in a Compact Disc) as shown in (a) in FIG. 2 or by irradiating laser beam for a period of time corresponding to length of a pit to be formed minus a predetermined length of time t0 as shown in (b) of FIG. 2.
Pit length or blank length can be set at various values depending upon consecutive times of occurrence of "1" or "0" of data to be recorded (e.g., 3T to 11T in a Compact Disc format). Since time length during which laser beam is irradiated increases with increase in the pit length, degree of heating of the recording film increases with increase in the pit length with a result that the recording film tends to be melted more quickly as the pit length increases. This phenomenon becomes more remarkable as heat conductivity of the recording film becomes larger relative to the linear velocity of the rotating disc.
For this reason, in prior art recording devices in which laser beam is irradiated for a period of time corresponding to the pit length regardless of magnitude of the pit length as shown in (a) or (b) in FIG. 2, the pit length of a pit which is actually formed becomes increasingly larger than a preset value as the pit length increases as shown in FIG. 3 (on condition that the length of blank immediately before the pit remains the same) even if power of laser beam is adjusted so as to obtain a preset pit length at, e.g., a pit of 3T. As viewed in eye patterns of reproduced signal waveforms shown in FIG. 4, eye patterns of reproduced signal waveforms in which blanks of 3T are formed respectively immediately after pits of 3T to 11T show that amplitude of the blank of 3T decreases as the pit length increases. This indicates that the actual pit length becomes increasingly larger than a preset value as the pit length increases. This causes jitter in a reproduced signal with resulting occurrence of errors and deterioration in the signal-to-noise ratio in the reproduced signal.
The length of a pit varies also depending upon the length of a blank immediately before the pit (hereinafter referred to as "immediately preceding blank length"). As immediately preceding blank length decreases, degree of influence of heating made in forming an immediately preceding pit on forming a next pit increases with a result that the recording film is melted more quickly in forming the next pit. Accordingly, even if radiation time of laser beam is the same, pit length of an actually formed pit increases as the immediately preceding blank length decreases.
FIG. 10 shows an example of variation of pit length of an actually formed pit depending upon immediately preceding blank length with respect to pits of 3T, 7T and 11T. In this figure, combination of pit length and blank length of the same time length is taken as reference Ref for each pit length and deviation from the reference Ref for each pit length is plotted. As will be apparent from the figure, as immediately preceding blank length decreases, the recording film is melted more quickly due to influence of heating made in forming a preceding pit so that the pit length increases despite the same radiation time of laser beam. This causes jitter in a reproduced signal with resulting occurrence of errors and deterioration in the signal-to-noise ratio in the reproduced signal.
The variation in immediately preceding blank length causes variation not only in pit length but also in relationship between a radiation start position and a pit starting end position of a pit formed by the radiation. That is, as immediately preceding blank length increases, distance between the radiation start position and the pit starting end position increases. This is because influence of heat from an immediately preceding pit decreases as the immediately preceding blank length increases with a result that it becomes harder to melt the recording film.
For this reason, if it is assumed that radiation is started at a predetermined radiation start position irrespectively of immediately preceding blank length, the pit starting end position is increasingly deviated behind as the immediately preceding blank length increases so that a correct blank length cannot be obtained. Since pit length and blank length have the same data weight for recorded data, this also causes jitter in a reproduced signal.
It is, therefore, an object of the invention to reduce errors in pit length and blank length and thereby decrease jitter in a reproduced signal for reducing errors and improving the signal-to-noise ratio in the reproduced signal.